The study, which appeared in a recent issue of the journal
Clinical
Biomechanics, is the first to examine what happens to
muscle oxygenation over a full workday.

Despite the fact that the study participants were performing
the same job at the same pace all day, their back muscles needed
more oxygen as the day went on. Taking a half-hour lunch break
helped their muscles recover from the morning's exertion, but
once they started working again, their oxygen needs rose steeply
and kept climbing throughout the afternoon.

"That was alarming to us, because it means that their
muscles were becoming fatigued much faster during the afternoon,
and we know that fatigue increases the risk of back injury,"
Marras said.

Two 15-minute breaks, one mid-morning and the other mid-afternoon,
helped muscles recover a little, but not as much as the half-hour
lunch.

"This tells us two things," Marras said. "First,
rest is good -- a half-hour break does a good job of helping
muscles recover. But it also tells us that people are especially
at risk for back injury at the end of the day, and the only way
to counteract that effect is with more breaks as the day goes
on."

"Because the oxygen
demand at the end of the day was so much higher, that's when
we'd expect people to get hurt on the job," Marras said.
"And the data I see coming out of industry bear that out
-- people tend to hurt their back toward the end of a shift."

Ten people participated in the study, six of whom had at least
one year's experience in a job that requires lifting, such as
stocking store shelves. The other four were considered novice
lifters.

One person would lift a box from a waist-high stand and set
it on a chest-high conveyor belt in Marras' lab, which simulates
a typical shipping center. The box then traveled down the belt
to the other person, who would lift it and set it on another
conveyor belt. They lifted boxes of three different weights --
two pounds, 11 pounds, and 26 pounds -- and they worked for the
entire eight hours, except for the half-hour lunch break and
the two 15-minute breaks.

Each person wore a Lumbar Motion Monitor, a device that Marras
designed to measure the movement of the spine. They also wore
oximeters on their lower back -- devices that measured the oxygen
level of their muscles through the skin. Just like the pulse
oximeters that doctors clip to a patient's finger, these sensors
use an LED light to detect the flush of color to the skin when
blood carries oxygen to the tissues underneath.

Study coauthor Gang Yang, a medical doctor who is now earning
a doctoral degree in biomechanics at Ohio State, said that the
researchers' top priority was making sure the subjects didn't
grow fatigued enough to become injured during the study. The
heaviest box they had to lift, 26 pounds, weighed less than half
as much as the loads that some workers are routinely required
to lift in industry.

In Clinical Biomechanics, the researchers detailed the oxygen
levels in the muscles of the typical study participant. During
the first two hours of lifting, the oxygenation level gradually
increased until it reached 11 percent above resting level. During
hours two to four, it rose to 13 percent. It returned to resting
level during lunch, but immediately rose 11 percent as the people
started lifting again during hours four to six. During the last
two hours of the day, oxygenation rose to its highest level --
16 percent above resting level.

"Because the oxygen demand at the end of the day was
so much higher, that's when we'd expect people to get hurt on
the job," Marras said. "And the data I see coming out
of industry bear that out -- people tend to hurt their back toward
the end of a shift."

Meanwhile, data from the Lumbar Motion Monitor showed that
the participants used their muscles differently as they became
fatigued -- a finding that meshes with Marras' previous work.
He's found that when people's back muscles begin to hurt, they
tense up, and try to lift with other muscles that don't hurt
as much.

"Now because of this study, we have a clinical reason
for why that's happening. It's because the muscles are becoming
fatigued, because they have such a high demand for oxygen,"
Marras said.

Tensing muscles prevents proper blood flow, so the muscles
are even further deprived of oxygen. And using different muscles
to lift may lessen pain at first, but it increases the stress
on the joints and the spine, and increases risk of serious injury
in the long run.

"When that happens, it's like the muscles fight each
other," Marras said. "You have back muscles that fight
the abdominal muscles, and when they both contract, it's like
a seesaw effect, except you're pulling down on both ends, and
your spine is in the middle."

The researchers found that participants who had never lifted
for a living let their muscles tense up during the study. Their
muscles also needed more oxygen than the experienced lifters,
who generally relaxed their muscles and used the proper muscles
for lifting.

"The bottom line is that it's much more costly from a
physiological standpoint for novices to do the same work as experienced
people," Marras said.

Low back pain is the most common and most costly musculoskeletal
disorder in the American workplace. According to the U.S. Bureau
of Labor Statistics, in 2002 there were more than 345,000 back
injuries requiring time away from work. A 2004 study by Harvard
Medical School and Massachusetts General Hospital found that
back pain results in over 100 million lost work days per year.
And a Duke University Medical Center Study found that in 1998,
total health care expenditures incurred by people with back pain
in the United States reached $90.7 billion.

Taking half-hour breaks instead of the standard 15 minutes
might help reduce back injury, Marras said, although he acknowledged
that such long breaks might not be practical in industry. He
pointed to other studies, however, which showed that shorter
breaks, taken more frequently, have a similar positive effect.

Marras and Yang's coauthors on the study included former graduate
students Anne-Marie Chany, now at Columbus Children's Hospital,
and Julia Parakkat, now at Wright-Patterson Air Force Base; and
Deborah Burr, associate professor of biostatistics at the University
of Florida.